These teachings relates to switching power supplies (switching converters). These devices are used to efficiently transform to voltage and currents at on level to voltage and current of a different level. Switching converters are particularly important when either high power or battery operation require high efficiency. Switching converters are pervasive throughout many consumer products they are in almost every ball everyday items such as cell phones PDAs personal computers extra. A key feature of the switching power supplies is its small size and low cost, which is achieved through efficient design.
Switching converters are used to convert an input DC voltage to an output DC voltage. Such converters may step down (buck) or step up (boost) the input DC voltage (buck-boost converters are also possible),
Conventional power supplies use Pulse Width Modulation (PWM) modulation to control the power devices used in converters. One type of switching converter is a synchronous buck converter. This converter typically has a controller, driver, a pair of switches, and an LC filter coupled to the pair of switches. The controller provides a control signal to the driver which then drives the pair of switches, e.g., a high side switch and a low side switch. The driver alternately turns each switch ON and OFF thereby controlling inductor current and the output voltage of the DC to DC converter. Such controllers typically utilize a pulse width modulated signal to control the state of the high and low side switches.
One of the ways to improve the size and cost of switching converters is to optimize the size of the external passive components. This is achieved by optimizing both the switching frequency and control loop.
With the advent of deep sub-micro CMOS, power supplies with very low voltage, high tolerance and high currents are required. As a result of passive filter components have to be scaled to a very low impedance, and in particular the output capacitance is scaled to be of high quality and large value. This capacitor dominates the size and cost of the switching converters for sub-micro CMOS. In general, the smaller the capacitor, the lower the cost.
There is a need for control techniques that allow the output capacitor to be reduced.
A typical voltage mode controller, shown in FIG. 1, consists of a fixed compensator controlling the pulse width modulator. The input to the fixed compensator is an error signal that is the difference between the desired reference voltage and the output voltage. The typical form of the fixed compensator with a PID controller (proportional derivatives and integral control). This type of control allows for relatively fast performance, provided the converter stays in either discontinuous conduction or continuous conduction. If both action modes are possible the compensator bandwidth has to be lowered to ensure a stable compensation. Because the duty cycle to output voltage transfer function is different between continuous conduction and discontinuous conduction.
One solution to this problem, shown in FIG. 2, is to add an inner high speed current feedback loop. This is similar to rate control for servo mechanisms. The inner current loop linearizes the control of the inductor current. Thus that the current command to output voltage is the same, independent of conduction mode so the compensator can be the same for both operating modes.
This technique is limited however. The primary limitation comes from the fact The inner current loop has limited control authority, ad as such, has rate saturation in the inner part of the loop. This is caused by the finite power supplies that can be applied to inductor. This rate saturation causes loop and instability for high bandwidth. Thus, the current mode controller must have a lower bandwidth in order to be stable particularly at high and low duty cycles. Thus, in some conventional methods, the response of the system to disturbances or other changes can be slower than is desirable.
There is a need for control methods that allow high bandwidth operation.